The expansion of modern agriculture has relied on the massive use of synthetic nitrogen fertilizerswhich have allowed us to feed a constantly growing global population. However, this same green revolution has brought with it a problem that can no longer be ignored: the enormous burden of greenhouse gases and air pollutants associated with both the manufacture and use of these products.
Today we know that indiscriminate fertilization has a very high hidden cost: emissions of nitrous oxide (N₂O), ammonia (NH₃), nitrogen oxides (NOx), the concentration of CO₂ and fine particlesGroundwater and surface water contamination; loss of biodiversity; and a significant impact on human health. The good news is that we have sufficient scientific knowledge, measurement technologies, and management alternatives to drastically reduce these emissions if combined. good policies, agricultural innovation, and changes in habits.
Fertilizers and climate change: why nitrogen is key
Agriculture and livestock farming are responsible for around 30% of global greenhouse gas emissionsAnd within that percentage, fertilizer use, manure management, and livestock feed play a significant role. The sector not only suffers the consequences of global warming (droughts, extreme weather events, new pests), but also It fuels the problem with its own emissions.
After carbon dioxide (CO₂) and methane (CH₄), the major climate factor linked to fertilization is... nitrous oxide (N₂O)It is a gas that is much less abundant in the atmosphere than CO₂, but with a global warming potential about 300 times greater and a lifespan exceeding a century. Furthermore, it contributes to ozone layer depletionThus, its impact goes beyond the climate and also affects the ultraviolet radiation that reaches the Earth's surface.
Before the Industrial Revolution, the N₂O balance was relatively balanced: microorganisms from natural soils and oceans They emitted amounts similar to those that natural sinks were able to absorb. The surge occurred when the world's population and demand for food grew, along with the massive production and application of chemical fertilizers, the increase in livestock, and the intensification of agriculture. In the last four decades, human activities have driven N₂O emissions to around a 40%, and it is estimated that Agriculture accounts for approximately 74% of anthropogenic emissions of this gas.
Within that agricultural bloc, the synthetic nitrogen fertilizers They are responsible for around 70% of the sector's N₂O emissions, while the livestock manure management It contributes approximately the remaining 30%. To this we must add an emerging source: the intensive aquacultureespecially in countries like China, where fish farming has increased 25-fold in recent decades, also generating nitrogen flows that end up in the aquatic environment and the atmosphere.
How are N₂O, NH₃ and NOx emissions produced from fertilizers?
The common link between the different gases involved in fertilization is the reactive nitrogenThe plant only manages to utilize, on average, between one 30% and 50% of the nitrogen supplied with synthetic fertilizers; the rest is lost in the form of nitrates that seep into the water, ammonia that volatilizes, or nitrogen family gases that pass into the atmosphere.
Two major microbial processes occur in the soil that explain a large part of these emissions: nitrification and denitrificationDuring nitrification, specialized bacteria oxidize the ammonium (NH₄⁺) to transform it first into nitrites (NO₂⁻) and then into nitrates (NO₃⁻), which are the forms most easily assimilated by plants. Small amounts of [unclear - possibly "products" or "energy"] may be generated in this process. N₂O as a by-product.
Denitrification takes place when The oxygen content in the soil decreases.This is common after heavy rains, abundant irrigation, or in compacted soils. Under these conditions, other microorganisms use nitrate as an electron acceptor and reduce it to nitric oxide (NO), nitrous oxide (N₂O), and finally molecular nitrogen (N₂)which is harmless and makes up most of the air. The problem is that a significant fraction of that flow remains in the intermediate form N₂O, which escapes into the atmosphere and reinforces the greenhouse effect.
When excessive doses of nitrogen fertilizer are applied, the soil becomes saturated and The excess nitrogen can no longer be retained by the systemOne part is transformed into gaseous ammonia (NH₃)This is especially true under conditions of high pH, high temperatures, and poorly incorporated surface soils. This NH₃ volatilizes, forms fine particles (PM₂, PM₅) when it reacts in the atmosphere, and can travel long distances before settling, affecting air quality and contributing to the eutrophication of distant ecosystems.
Another percentage of that nitrogen surplus is oxidized in the atmosphere forming nitrogen oxides (NOx: NO and NO₂)These gases participate in the formation of tropospheric ozone and photochemical smog, and are considered key pollutants in air quality strategies. Along with CO₂ and CH₄, NOx reinforce the greenhouse effect and climate changein addition to directly affecting the human respiratory system.
Emissions by country and global trends
Emissions from fertilizers and manure are not uniform across the planet; they depend on various factors. economic, agricultural, demographic and politicalEmerging economies that have opted for a strong increase in agricultural productivity, such as China and IndiaThey show a clear upward trend in their N₂O emissions in recent decades in order to meet the growing demand for food.
China has become the main producer and consumer of chemical fertilizers of the world. The implementation of specific plans to limit the growth of fertilizer consumption, such as its “zero growth” program by 2020, has helped to moderate some of these emissions by improving the efficiency of nitrogen use. However, in parallel, the Industrial emissions of N₂O Those linked to the production of fertilizers and other chemicals remain very significant.
In regions like Brazil and Indonesia Another factor is the clearing and burning of forests to gain land for agriculture and livestock. This transformation of land use increases nitrogen losses from natural sources and amplifies greenhouse gas emissions, combining the CO₂ released by deforestation with the N₂O derived from fertilization and livestock management.
The African continent presents a dual nature. On the one hand, there are still large areas where Food production could be increased without needing to increase nitrogen fertilization, by first improving water, soil, and crop management. On the other hand, some North African countries have tripled its emissions in the last two decades, mainly due to the growth in the number of livestock and the intensification of animal production.
In contrast, Biasi's European Union, Japan and South Korea They have achieved a significant reduction in their anthropogenic N₂O emissions over the past 40 years. Much of this decrease stems from measures in the chemical industrywhich has incorporated N₂O abatement technologies into the production processes of nitric acid and other compounds. Agriculture in these regions has become more efficient in its nitrogen use, but emissions from the direct application of fertilizers and manure have only decreased slightly and are tending to stabilize.
Impact of the fertilizer industry on the atmosphere
The climate footprint of fertilizers is not limited to the moment they are applied to the field; it begins much earlier, in the manufacturing of ammonia and nitrogen fertilizersAt the beginning of the 20th century, the development of the Haber-Bosch process made it possible to fix atmospheric nitrogen (N₂) by combining it with hydrogen to obtain liquid ammonia (NH₃) on an industrial scale. This represented a spectacular leap in agricultural productivity, but it also opened the door to a sharp increase in associated emissions.
Producing a nitrogen fertilizer involves high energy consumption, typically based on fossil fuels, and the emission of CO₂ and other gases. It is estimated that manufacturing 1 kg of nitrogen fertilizer can generate around 7 kg of CO₂If, on the other hand, the industry adopts the Best Available Techniques (BAT) recommended at the European level, that figure can be reduced to around 3,6 kg of CO₂ per kilogram of nitrogenThat is, practically half the emissions to obtain the same product.
During production processes, reactions with acids, high pressures and high temperatures, manufacturing plants also release soot, dust, and a mixture of polluting gasesSulfur oxides (SOx), unreacted ammonia, nitric oxide (NO), nitrogen dioxide (NO₂), and volatile organic compounds. This combination directly affects local air quality, surrounding ecosystems, and the health of people who live or work near these facilities.
For this reason, fertilizer factories are subject to strict environmental regulations These regulations require emissions controls, the installation of purification systems, and the implementation of prevention and maintenance measures. Even so, regulatory pressure varies by region, and part of the global problem is concentrated in countries with less stringent regulations or lower levels of compliance.
Types of fertilizers and their relationship to emissions
From an agronomic point of view, fertilizers are classified according to the origin of their nutrients into organic, mineral fertilizers or synthetic, biofertilizers and organic-mineral fertilizersEach type behaves differently in the soil and, therefore, has a different environmental footprint.
The organic fertilizers These include manure, compost, and plant debris. They provide carbon-rich organic matter that soil microorganisms slowly break down, gradually releasing nutrients. This process improves the structure, porosity and water retention capacity of the soil, and helps maintain its fertility in the long term. They are the basis of organic farming and tend to generate fewer rapidly lost nitrogen surpluses, although they can also emit N₂O and NH₃ if mismanaged.
The synthetic or mineral fertilizers They come from industrial chemical processes that transform salts, gases, and rocks into forms available to plants. They primarily provide nitrogen, phosphorus, and potassium (NPK)These fertilizers are supplemented with micronutrients such as zinc, iron, manganese, or copper. Their agronomic advantage is that they provide nutrients in readily available forms, allowing for rapid crop responses and high yields. Their weakness is that, if used excessively or without careful dosage and application timing, generate significant surpluses which result in emissions into the atmosphere and water pollution.
The biofertilizers They utilize live microorganisms (bacteria, fungi, cyanobacteria) that stimulate biological processes in the soil, improving nutrient availability and root absorption. They promote biological nitrogen fixation, phosphorus solubilization, and more efficient use of applied fertilizer, without leaving large amounts of free reactive nitrogen that could be converted into N₂O or leached nitrates.
The organic-mineral fertilizers They combine the mineral fraction with organic matter of animal or plant origin. In this way, the speed of action of the chemical fertilizer and the capacity to improve the soil of organic matter, reducing to some extent the risk of sudden nitrogen losses and improving the resilience of the agricultural system.
Which fertilizers pollute the most: urea, ammonium nitrate, and others
Although all fertilizers have their own particular environmental footprint, not all have the same impact. Among the minerals, those containing u They are generally considered more problematic than those based on ammonium nitrate when the complete cycle and N₂O emissions in the field are analyzed.
Ammonium nitrate is obtained from ammonia and nitric acid, and its footprint depends primarily on the energy consumption of the process, from the hydrogen source used to produce ammonia and from the N₂O emissions during nitric acid manufacturingIn urea production, some of the CO₂ generated in ammonia synthesis is incorporated into the urea molecule itself, so the direct CO₂ emission at the factory may seem smaller.
However, once urea is applied to the soil, that carbon is also released in the form of CO₂, and furthermore the process of hydrolysis and nitrification of urea It tends to generate higher N₂O emissions in the field than nitrate fertilizers. In practice, considering the entire life cycle, Urea fertilizers tend to have a greater global climate impact to that of ammonium nitrates.
Other fertilizers with a significant environmental footprint include ammonium sulphate or potassium chlorideThis is due both to the emissions linked to their production and their effect on the soil (acidification in the case of ammonium sulfate, salinization in the case of potassium chloride). In contexts where the aim is to reduce pollution associated with mineral fertilization, it is usually recommended to prioritize Nitric fertilizers with a smaller footprint and maximize the efficiency of its use.
Carbon footprint studies show that emissions during the fertilizer manufacturing are comparable, in magnitude, to the emissions generated after its application through nitrification, denitrification, and volatilization processes. In other words, the pollution is split almost 50/50 between what happens in industry and what happens on the plot of land.
Air, water and soil pollution linked to fertilizers
The problem of excess reactive nitrogen has multiple facets. In the atmosphere, nitrogen that doesn't reach the plant returns as N₂O and NOxAmmonia, gases with a very high warming potential that intensify the greenhouse effect. The emitted ammonia forms fine particulate matter (PM₂, PM₅), combines with other pollutants, and affects the air quality in rural and urban areasEnvironmental authorities systematically monitor NOx, volatile organic compounds, SO₂, NH₃, and fine particulate matter, and while many of them show downward trends, in countries like Spain the Ammonia emissions have rebounded Recently, this has been largely due to increased nitrogen fertilization and livestock intensification.
In water, runoff and leaching carry nitrates and nitrites into aquifers, rivers, lakes, and seas. This massive influx of nutrients triggers processes of eutrophicationwith harmful algal blooms, decreased dissolved oxygen, and the appearance of dead zones where aquatic life cannot survive. Furthermore, nitrates in drinking water pose a health riskespecially for babies and vulnerable people.
In the soil, excessive applications of synthetic fertilizers alter the soil microbiota and disrupt the balance between nitrogen, phosphorus, and carbon. This can lead to soil acidification, loss of organic matter, and a weakening of its physical structure. In the long run, far from improving fertility, the overuse of chemical fertilizers can degrade the soil and make crops more dependent of external inputs to maintain yields.
At the ecosystem scale, the deposition of atmospheric nitrogen in forests, grasslands, or protected areas alters the species composition, favoring those that best utilize this extra input and reducing the biodiversityNitrogen, which in the right amount is essential for life, becomes, when there is too much, a major environmental stressor.
Consequences for human health
The presence of nitrogen compounds in the air we breathe is not a minor issue. nitrogen oxides, ammonia and fine particles The effects of these particles on the respiratory system, exacerbate cardiovascular diseases, and are linked to immune system disorders. PM₂,₅ particles can penetrate deep into the lungs and even cross the alveolar barrier, with significant short- and long-term consequences.
It is estimated that a significant portion of the premature deaths linked to air pollution It is associated with PM₁₀ and PM₂₅, whose formation involves agricultural ammonia. The direct toxicity of NO₂ and other gases is compounded by the combined effects of other urban pollutants, creating a cocktail that strains the healthcare systems of many regions worldwide.
In countries with highly intensified agriculture, such as some areas of India, the continued and increasing use of fertilizers and pesticides It has been linked to an increase in respiratory illnesses, endocrine disorders, neurological problems, and a higher incidence of certain types of cancer, such as bladder, ovarian, and lymphoma cancers. Farmers and their families are on the front line of exposure, both through direct contact during application and through water and air contamination in their environment.
Added to all this are the risks of acute episodes of pollution In the event of leaks or accidents at fertilizer plants, where high concentrations of ammonia, NOx, or other hazardous compounds may be released, constant monitoring and early detection are therefore key tools for preventing emergencies and reducing chronic exposures.
Emissions monitoring: sensors and advanced techniques
In order to manage greenhouse gas emissions and pollutants associated with fertilizers, it is first necessary to measure them accuratelyIn industry, an effective strategy consists of deploying a perimeter ring of sensors Located around production plants, this system is capable of recording real-time concentrations of NO, NO₂, NH₃, SOx, and volatile organic compounds. This information allows for the rapid detection of leaks, process optimization, and compliance with legal limits.
In agricultural areas, air quality station networks powered by solar energy They allow monitoring the evolution of nitrogen compounds during fertilization campaigns. This data helps identify the most critical times and conditions for emissions, adjust fertilization practices, and evaluate the impact of new technologies or regulatory changes.
In addition to conventional environmental sensors, the nuclear and isotopic techniques They provide powerful tools for tracing the origin and destination of nitrogen. The use of the stable isotope nitrogen-15 makes it possible to determine what fraction of the N₂O emitted comes from applied fertilizer, manure, or natural soil reserves. Similarly, carbon-13 is used to study the carbon sequestration in the soil and to assess how practices such as crop rotation, no-till farming or the use of biochar influence the soil's capacity to store CO₂ in the long term.
In the case of livestock farming, the analysis of long-chain hydrocarbons and carbon-13 The presence of nutrients in plants ingested by ruminants and in their feces helps to accurately estimate grazing consumption, making it easier to design more efficient supplementation strategies and reduce energy leaks and emissions associated with animal production.
Strategies to reduce fertilizer emissions
Addressing the challenge of fertilizers and their emissions requires a combination of policy measures, technological innovations, and behavioral changes at all levels. It's not about giving up fertilization, but about using nitrogen much more efficiently and prioritizing less polluting sources and practices.
In industrial production, the widespread adoption of N₂O abatement technologies In the manufacture of nitric acid and other intermediates, as well as improving energy efficiency and using less carbon-intensive energy sources, are “easy wins” that can almost completely eliminate industrial N₂O emissions. Many countries have already done so, leaving a few large emitters responsible for most of the remaining problem.
In the field, the practices of intelligent agronomic management These are essential: adjusting fertilizer doses to the actual needs of the crop, choosing the most appropriate time and method of application, avoiding applications before episodes of heavy rain, incorporating the fertilizer into the soil to reduce volatilization, and combining mineral fertilizers with organic amendments that improve the structure and nutrient retention capacity of the soil.
The partial replacement of synthetic fertilizers by organic fertilizers, biofertilizers and organic-mineral fertilizers It helps to reduce excess reactive nitrogen and increase soil organic matter. At the same time, livestock farming can reduce its emissions by improving... animal diet, manure management and the treatment of manure, for example through anaerobic digestion with biogas recovery.
Consumers also have room to maneuver: increasing the proportion of vegetarian food In our diet, minimizing food waste, composting organic waste, and reducing fertilizer use in gardens and lawns helps to ease the global pressure on the nitrogen cycle. It's not essential to adopt a completely vegan diet to notice the effect; gradual reductions in the frequency and amount of meat and dairy consumption are enough to generate a noticeable change in our food-related nitrogen footprint.
In high-value, high-impact crops such as indoor cannabisGiven that cannabis cultivation combines intensive fertilizer use with enormous electricity consumption for lighting, climate control, and CO₂ generation, improving energy efficiency (for example, with LED lighting) and committing to organic fertilizers and more sustainable management techniques are especially urgent. Some estimates have equated 1 kg of cannabis produced under certain conditions with several thousand kilograms of CO₂ emitted, figures that demonstrate the potential for improvement in these types of systems.
Taken together, this entire network of sources, processes, and solutions paints a picture in which nitrogen fertilizers are both an essential tool for feeding the world and one of the Gordian knots of climate change, air quality, and ecosystem healthMoving towards truly sustainable agriculture and livestock farming involves rethinking how we produce, distribute and use these fertilizers, relying on science, measurement technology and a set of good practices that allow us to continue producing food without further burdening the atmosphere, water and soils with more nitrogen than the planet is capable of managing.
